The present invention relates to an improved flex (or TAB) product suitable for silicon carrier or other types of chip carrier applications, wherein the flex reliability problems caused for example by Cu dendrite growth and lead bending during power and thermal cycling are substantially reduced or eliminated. More particularly, the invention embodies a number of coatings for use in such products and diverse methods of making and using same.
In the first embodiment, the entire flex containing openings (or windows) in the substrate comprising polyimide, fabricated by one of the four methods detailed hereinafter, is manufactured in such a way that the coating and substrate are patterned and etched at the same time.
The second embodiment of the invention relates to a liquid or colloidal coating that is applied by, for example, silk screening on the inner/outer lead areas on a flex tape. The properties of the liquid used in combination with the windows in the substrate allow very precise coating with a wide latitude for alignment.
In the third embodiment of the present invention, the flex product, used as the interconnection between carrier and printed circuit board, is coated with a thin layer of low stress material characterized by a low product of modulus and thermal expansion coefficient. The coating prevents lead breaks and electrical shorts between adjacent leads caused by excessive lead bending upon thermal cycling.
In a fourth embodiment of the invention, a thin layer of low stress coating material is applied at the appropriate location on the outer lead area of a flex to seal off the area around each exposed lead with a polymer means.
According to a standard flex (or TAB) fabrication process for chip carrier applications, the lead windows in the polyimide film substrate are created after the plating of the metal pattern. This is typically achieved by wet etching the polyimide substrate through a resist window on the polyimide using a caustic etch agent (e.g., aqueous KOH solution). Following formation of the window, a standard practice is to coat the metal pattern with a thin layer of Au. In certain cases, the Au coating meant to protect the metal pattern does not prevent flex reliability problems such as Cu dendrite growth during power-up.
According to the prior art, one method to solve these reliability problems is to spray coat the bulk of the flex over an area between the inner and outer lead area with a high stress coating material such as a filled epoxy (e.g., Scotchcast) before bonding of the inner and outer leads to substrates (e.g., silicon carrier and printed circuit board) with matching metal pads.
One reason that the aforementioned high stress coating cannot be applied directly over the inner/outer lead areas is the difficulty involved in making a metal spray mask covering these areas with fine details, such that during spraying, the liquid coating material does not contaminate the critical bonding areas of these leads.
After bonding, a final coat is then sprayed over the areas containing bonded inner and outer leads.
The flex prepared by the method noted above, however, can suffer from the reliability problems such as Copper (Cu) dendrite growth over the inner/outer lead areas due to incomplete coverage of the flex by the final coat.
Also, during flex coating prior to bonding, damage can be inflicted on the partially supported Cu leads due, for instance, to the contact between flex leads and spray mask as well as other physical fixtures that can be in contact with the flex, which as a result can greatly reduce the yield of the flex coating step.
In addition, the partially supported leads on the flex over the inner/outer lead areas after bonding to the substrates can suffer from severe lead bending during power and thermal cycling, leading eventually to lead breaks and electrical shorts due to the high thermal stresses exerted by the high stress coating.
U.S. Pat. No. 4,209,355 discloses a composite tape product having an insulated layer with an aperture therein and a plurality of conducting leads extending in cantilevered fashion into the aperture.
U.S. Pat. No. 4,681,654 discloses a method for making flexible film substrates in a continuous tape format and describes the use of metallizing and etching polyimide layers to provide circuit patterns on the polyimide layer.
U.S. Pat. No. 4,721,994 discloses a semiconductor device lead frame wherein a dielectric polyimide layer is bonded to the surfaces of the inner leads.
None of these references discloses solutions to the problems associated with the flex coating mentioned in the prior art.
Finally demountable packages in a variety of electronic applications require that one of the connections between the chip and the base to which it is secured, known as the motherboard, be non-permanent, to allow removal and replacement of the package. In the case of a pad-on-pad (POP) module, the outer leads of the flexible connector generally rest on gold pads on the card, held in place by mechanical compression. This situation introduces new concerns about problems at the connection which were previously solved by some kind of permanent fixture or material. For example, the problem of metallic corrosion at the outer lead areas of the flex has been solved in the past by encapsulating these areas after bonding with a permanent polymeric coating. Clearly, some sort of non-permanent alternative must be found for the encapsulating coating, since corrosion protection is also an issue in the demountable package. The primary function of a coating is to prevent the introduction of contaminants or elements which could damage the part or create a damaging environment. Another way to achieve this would be to surround the sensitive area with a sealant which would keep out contaminants and moisture.
In accordance with the present invention, the task of protecting the flex from the problems as noted above, including the flex coating yield, Cu dendrite growth, lead breaks and electrical shorts, is achieved by the use of one or more of the embodiments of the present invention.
Following the application of the polymer coating, there are four methods in the first embodiment detailed below that can be used satisfactorily to create the lead window.
The first and preferred method in this embodiment by which the lead windows can be created in the substrate comprises developing the coating and etching the polyimide in a polyimide etchant using the coating as a permanent photoresist.
As a second alternative, the coating and polyimide are etched simultaneously or sequentially using an appropriate polyimide/coating etchant with the help of a resist. Generally in this instance, the coating should be etchable in the polyimide etchant.
As a third option, the coating is first etched using a solvent that does not etch the polyimide with the help of a resist and then the polyimide is etched using a polyimide etchant prior to resist removal. In this instance, the coating preferably should not be etchable in the polyimide etchant.
In the fourth option the coating and the polyimide are ablated using a laser through the use of masks.
The second and third embodiments noted above involve coating the flex after the windows have been created, but before bonding, using a suitable silk screening method with preferably a low stress, filled or unfilled silicone coating. In this instance, the mask can be designed so that the entire flex rather than just the bulk area of the flex away from the inner/outer lead areas is coated.
In addition to the properties and/or characteristics mentioned above, the polymer coating material for any of the embodiments mentioned above should preferably possess the following characteristics or properties: (1) good adhesion to the polyimide-metal pattern; (2) good gap-filling properties; (3) relatively thin and flexible so that the flex is drapable during joining; and (4) resistant to subsequent processing and testing conditions.
With respect to the first method in the first embodiment cited above, a polymer is used for the coating. A particularly useful polymer coating is a partially cyclized poly-cis isoprene such as KTFR from Kodak. It contains an average unsaturation of one double bond per 10 carbon atoms which is about 50% of the double bond content of the uncyclized polymer. The double bond may be present in uncyclized C5 H8 units and/or in six membered rings. A possible structure for the repeating unit is: 
The number average molecular weight of the polymer determined by osmometry is generally about 65,000xc2x15,000. Based on this value, the weight average molecular weight is about 121,700xc2x195,000. A gel permeation chromotographic (GPC) study that has been published relating to this polymer reports a number average molecular weight of 46,000 and a weight average molecular weight of 141,000. A useful sensitizer compound is 2,6-bis (p-azidobenzylidene)-4-methylcyclohexane.
For the second and third methods in the first embodiment, the polymer coatings that can conveniently be used comprise a polyimide film with a suitable adhesive (for example, Thermid from National Starch), a sprayed-on polyimide properly cured on the flex before etching or a partially cured polyimide dry film laminated and cured on the flex.
With respect to the fourth method in the first embodiment relating to ablating the coating and the polyimide using a laser, many polymers are suitable such as epoxies, acrylics and/or the chemical vapor deposited parylene which is not etchable by any solvent.
The low stress silicone coatings preferred in the second and third embodiments noted above include addition-cured based polymers such as SC-3613 (Emerson-Cuming), or JCR-6125 (Toray), optionally containing a non-electrically conductive filler, such as TiO2, or other similar materials.